The present invention relates generally to plasma processing equipment. In particular, the present invention relates to calibration and control of RF and microwave plasma processing equipment.
Radio frequency or microwave (hereinafter xe2x80x9cRFxe2x80x9d) plasma generation equipment is widely used in semiconductor and industrial plasma processing. Plasma processing supports a wide variety of applications, including etching of materials from substrates, deposition of materials onto substrates, cleaning of substrate surfaces, and modification of substrate surfaces. The frequency and power levels employed vary widely, from about 10 kHz to 2.45 GHz and from a few Watts to as much as 100 kW or greater. For semiconductor processing applications, the range of frequencies and powers presently used in plasma processing equipment is somewhat narrower, ranging from about 10 KHz to 2.45 GHz and 10 W to 30 kW, respectively.
Plasma processing equipment typically requires a precision RF signal generator, a matching network, cabling, and metrology equipment. In addition, precision instrumentation is usually required to control the actual power reaching the plasma. The impedance of loads associated with a plasma can vary considerably in response to variations in gas recipe, plasma density, delivered RF power, pressure and other parameters.
An RF supply, including a signal generator and matching network, can deliver power to the plasma in a number of ways, for example, via an antenna or sample holder. An antenna typically has a primarily inductive load impedance, with a smaller resistive component. In contrast, a sample holder (a xe2x80x9cchuckxe2x80x9d or xe2x80x9cbiasxe2x80x9d) typically presents a primarily capacitive impedance, also with a smaller resistive component.
Matching networks are typically positioned between the output of the RF generator and the input of the process chamber. The matching network provides a means of matching the output impedance of the generator to the input impedance of the process chamber. A matching network often includes elements such as variable capacitors and variable inductors to permit dynamic impedance matching of an RF generator to a changing load.
Most RF generators for plasma processing equipment are designed to have a standard fifty-ohm output impedance. A matching network can accommodate mismatches in impedance between the standard fifty-ohm output impedance of the RF generator and the input of the load. The mismatch can be exacerbated by a process chamber and plasma whose associated load can fluctuate over a large range of values.
The impedance mismatch can cause inefficient power deliver. The mismatch can also cause the power delivered to the plasma to vary, which can cause process inconsistency both within a chamber for successive substrates and among similar chambers. Thus, use of an impedance matching network can improve the efficiency of power transfer from a signal generator to a plasma vessel.
Components used in some plasma generation systems can present further difficulties in process characterization and control. For example, many systems utilize coaxial cables to connect an RF generator to an impedance matching network.
Determination of the power delivered to a reactive load (i.e., as presented by the plasma vessel) can be difficult and of limited accuracy. A standard operating method entails holding power delivered to the plasma vessel load constant. The power, however, is generally not well known because, for example, power is lost in the matchbox; the lost power is a complicated function of, for example, the positions of the vacuum variable capacitors in a matchbox plus a plasma vessel load having a nonlinear behavior.
An impedance probe can be placed between the matchbox and the plasma vessel to obtain a measure of power delivered to the plasma vessel. This approach has at least two disadvantages. First, an impedance probe can be very inaccurate when the phase angle between current voltage waveforms is high. A high phase angle typically is encountered for a highly reactive plasma vessel load. Second, impedance probes are typically too costly for production systems.
Various embodiments of the invention remedy many limitations encountered in measurement, calibration and control of prior art RF powered plasma systems. An RF plasma generation system, according to principles of the invention, can include an RF generator, impedance matching network, a signal probe that monitors the RF signal between the generator and the matching network, a plasma vessel, and a calibration database. The calibration database provides data that permits accurate determination, for example, of plasma vessel power consumption via RF signal parameter values collected by the signal probe. The calibration database is obtained from measurements collected from the impedance matching network and a calibration load that represents the behavior of the plasma vessel.
The system can provide accurate measurements of plasma vessel impedance and plasma vessel power consumption, even when the load associated with the plasma vessel is highly reactive. The system is particularly well suited to designs that include a variable frequency RF generator and an impedance matching network having an impedance that is fixed during operation of the system. In such a system, the frequency can be adjusted to obtain a minimum in reflected power.
Accordingly, in a first aspect, the invention features a method for operating an RF plasma generation system. The system can include an RF signal generator, an impedance matching network, a plasma vessel, and an RF signal probe that monitors the RF signal delivered from the RF signal generator to an input port of the impedance matching network.
The method includes causing an RF signal to be applied to the input port of the impedance matching network, monitoring a present value of at least one parameter of the RF signal associated with the input port of the impedance matching network, and causing the RF signal to be applied from an output port of the impedance matching network to an input port of the plasma vessel. The RF signal is monitored at a location along a pathway of the RF signal between the RF signal generator and the input port of the impedance matching network.
The method also includes providing calibration data associating values of the RF signal parameter with values of a characteristic of a load associated with the plasma vessel. A present value of the characteristic of the load associated with the present value of the parameter of the RF signal is determined by referencing the calibration data.
The impedance matching network has a fixed impedance. The impedance matching network can have a plurality of fixed impedances associated with a plurality of operating recipes of the RF plasma generation system.
In a second aspect, the invention features a method for calibrating an RF plasma generation system. The method includes providing a calibration load that represents the load associated with the plasma vessel. The calibration load has an input port in electromagnetic communication with an output port of the impedance matching network. A sequence of RF signals is applied to the input port of the impedance matching network thereby causing the sequence of RF signals to be applied to an input port of the calibration load. At least one parameter associated with the sequence of RF signals applied to the input port of the impedance matching network is determined, as is at least one characteristic of the calibration load responsive to the sequence of RF signals.
The power of the RF signals can be ramped over a range of power values associated with operation of the RF plasma generating system. A value of the impedance of the calibration load can be determined to characterize the calibration load, and the value of the impedance can be stored. At least one value of the impedance of the load, associated with a center frequency of the RF signal, a range of frequencies of the RF signal, and/or a range of temperatures of the calibration load, can be determined. The characteristic of the calibration load can be a power consumption of the calibration load.
In a third aspect, the invention features an RF plasma generation system. The system includes an impedance matching network having an input port to receive an RF signal from an RF generator, and an output port to deliver the RF signal to an input port of a plasma vessel associated with a load. The system includes an RF signal probe in electromagnetic communication with the input port of the impedance matching network to detect at least one RF signal parameter associated with the RF signal at the input port of the impedance matching network.
The system also includes a calibration storage unit. The unit stores calibration data that includes an association of values of the RF signal parameter with values of at least one characteristic of the load. A control unit determines an association between the detected signal parameter and a present value of the plasma vessel load characteristic by referencing the calibration data.
In a fourth aspect, the invention features an RF plasma generation system. The system includes an impedance matching network, a plasma vessel associated with a load, and a data storage unit storing calibration data.
The calibration data is created by a method that includes providing a calibration load that represents the load associated with the plasma vessel. The calibration load has an input port in electromagnetic communication with an output port of the impedance matching network. The method includes applying a sequence of RF signals to the input port of the impedance matching network, determining at least one parameter associated with the sequence of RF signals applied to the input port of the impedance matching network, and determining at least one characteristic of the calibration load responsive to the sequence of RF signals.